1. Field of the Invention
This invention relates to an angle sensor for detecting the rotation angle of a rotating axis, an angle-torque sensor for detecting the rotation angle and torque of a rotating axis, and an electric power steering unit using the angle sensor or angle-torque sensor.
2. Description of the Related Art
Steering sensors are used in an electric power steering unit for automobile in generating a motor torque according to a steering torque by the driver to assist the steering of tires. For example, such a steering sensor is disclosed in Japanese patent application laid-open Nos. 5-149805 and 6-102113. This steering sensor is a noncontact type sensor, detecting the twisting of torsion bar through a change in inductance, thereby obtaining the steering torque. Also, Japanese patent application laid-open Nos. 2001-91375 and 2001-91377 discloses an angle-torque sensor that a torque sensor is integrated with an angle sensor.
For the electric power steering unit, a torque sensor for detecting the steering torque is needed to control the motor torque. An inexpensive torque sensor is desired to lower the manufacturing cost of the electric power steering unit.
Also, for the electric power steering unit, an angle sensor, so called steering angle sensor, for detecting the angle of tires to the straight driving direction of car is needed other than the torque sensor. Because, when the driver releases his hands from the steering wheel while driving a car, the motor should be controlled such that the steering direction of the tires is automatically returned to the straight driving direction as done by a car with no electric power steering unit installed.
By using such an angle-torque sensor that a torque sensor is integrated with an angle sensor as disclosed in Japanese patent application laid-open Nos. 2001-91375 and 2001-91377, the electric power steering unit can have a reduced size.
A sensor is at present desired that can detect the absolute angle of steering axis to make three to four turns with high precision while offering a low manufacturing cost thereof.
In the sensor disclosed in Japanese patent application laid-open No. 2001-91375, a rotation angle of input axis is obtained from the output of first and second magnetic sensors to detect the intensity of magnetic filed in magnetic wave bar disposed on input axis, a steering torque applied to input axis is obtained from an angle difference between a rotation angle of input axis to be obtained from a detection signal of first magnetic sensor for detecting the intensity of magnetic field in magnetic wave bar disposed on input axis and a rotation angle of output axis to be obtained from a detection signal of third magnetic sensor for detecting the intensity of magnetic field in magnetic wave bar disposed on output axis, and its principle is that a fraction of one turn in steering axis is detected as one cycle. In general, since the steering wheel rotates more than one turn clockwise or counterclockwise, the angle sensor is needed to detect an absolute angle of 720° or more. However, the sensor disclosed in Japanese patent application laid-open No. 2001-91375 lacks a precision in detecting the absolute angle since it uses the above principle.
The sensor disclosed in Japanese patent application laid-open No. 2001-91377 is capable of rotating with input axis and sliding in the direction of input axis, a sliding member to which magnets with different polarities are alternately attached on the circumference is fitted into input axis, a change in flux of magnet according to the sliding and rotation in the axis direction of the sliding member is detected by a first and second magnetic sensors, and a steering torque applied to input axis and a rotation angle is thus detected. However, the sensor uses the sliding member with such a complicated structure and, therefore, the manufacturing cost becomes expensive.